U.S. Pat. No. 3,552,518 (referred to subsequently as '518 and incorporated here by reference) shows an antitheft device utilizing the parking brake of a vehicle. As shown, for example, in '518, a parking brake will typically include a mechanism which can lock and release the brake. In '518, unlocking is accomplished using vacuum from the motor, but it is more common that unlocking is accomplished by manual movement of some release. In '518, means is provided which interferes with the unlocking, by selectable blocking of the vacuum line.